1. Field of the Invention
This invention relates to canisters for containing a bed of particles and more particularly but not exclusively to a canister for containing a bed of inorganic sorbent particles being a molecular sieve bed providing oxygen enriched air in an aircraft on-board oxygen generating system.
2. Description of the Prior Art
An aircraft on-board oxygen generating system (OBOGS) is described and illustrated in EP-A-0129304. The system illustrated in FIG. 1 of this reference has three canisters filled with suitable molecular sieve sorbent particulate material in provision of three so-called molecular sieve beds. Each canister has a weir plate extending internally of the canister from one end thereof and terminating short of the opposite end. In operation supply fluid, in this case air, enters one end of the canister at one side of the weir plate, flows through the molecular sieve material parallel to the weir plate, is turned around at the opposite end of the weir plate to return upon itself through tire sieve material on the opposite side of the weir plate before exiting the canister as product fluid, in this case oxygen-enriched air.
A problem encountered with the use of sorbent particles in such an installation is that of generation of sorbent dust which can contaminate the product fluid leaving the molecular sieve beds. Such sorbent dust can be generated if the sorbent bed becomes fluidized, i.e. if the particles of sorbent are moved by the pressurised fluid passing through the bed, causing collision with and/or abrasion against one another, generating the dust.
It has been proposed to avoid fluidization by compressing the sorbent particles using a Belleville spring device bearing against one end of the bed, however, a problem with such an arrangement is that with sieve bed settlement, the spring eventually runs out of travel so that further fluidization reduces the compressive load thereby resulting in dust generation.
A proposed solution disclosed in U.S. Pat. No. 4,665,050 immobilizes the inorganic sorbent particles by binding them to each other with a polymeric binding material; however, it has been found that in use beds of such immobilised particles are less efficient than a bed of "free" particles. In particular, the volume flow of product fluid is reduced so that to obtain a given volume flow a larger bed must be provided if the sorbent particles are immobilised and this may be unacceptable in an aircraft OBOGS where space required for housing the system must be kept to a minimum.
An object of the present invention is the provision of a new or improved canister for containing a bed of particles which overcomes or reduces the aforementioned problems.